In the heavy-duty engine market a number of changes are being made to comply with recent and prospective emissions standards. As the construction of the engine changes, engine designers must nevertheless maintain a robust engine design with a sufficient level of dependability. One of the more prominent changes being implemented is the increase of the power rating of the engine. To achieve higher horsepower, many engine designs require a higher boost. Increasing the required amount of boost, in turn, puts a greater demand on the valve seal. Many of these engines are increasing their boost by up to 50-60 psig, which is a far greater pressure than most bonded valve seals can handle while being properly retained on a valve guide. For such cases, an integral valve seal with a metal retainer is normally recommended.
In order to put an integral valve seal on a valve, the valve seal must cover the valve guide that protrudes from the deck of the cylinder head. However, some such valve guides exceed 50.8 mm. (2.0 in.) in height, a height at which metal stampings become very difficult to produce. Moreover, engine designers are reluctant to reduce the height of the valve guide to address the limitations of the seals.
Further, valve seal designs that attempt to address the inherent problems must meet a number of rigorous requirements to function properly. Unitary molded plastic valve seals are known to the industry. However, conventional unitary molded plastic sleeves are not generally suited to withstand the heat and high pressures of a heavy-duty valve spring opening and closing repeatedly over the course of the intended life of the cylinder head.
Likewise, the prior art discloses the use of rigid reinforcing members at various portions of the valve stem seals. However, such reinforcements provide their own inherent limitations. For instance, reinforcements are not always formed integrally with the valve seal; can be difficult to install; and often do not fully provide the desired physical/mechanical functions, for example, by acting as a spring seat between the valve seal retainer body, valve spring, and engine head.
Therefore, a need exists in the industry for an improved plastic valve seal that can be incorporated into a combustion engine using conventional assembly techniques and handle increased environmental heat and pressure, yet still provide a desired level of performance throughout the intended life of the part.